Conventional US production motor vehicles today include fuel systems in which a canister containing an adsorbent bed of activated charcoal granules is used to recover fuel vapors that would otherwise be lost to atmosphere. The fuel vapors so recovered include those produced from the carburetor bowl in conventionally aspirated engines, and the vapors produced in the fuel tank as it sits, the so called diurnal losses, which vapors must be vented to control tank pressure. Instead of venting the diurnal tank losses to atmosphere, they are now vented to and stored in the canister, from which they are later purged and sent to the engine air intake to be burned. Another not inconsiderable source of vapors from the fuel tank are those that are displaced from the empty tank volume during refueling. These have to be vented somewhere, in order to prevent the pressure in the tank from building and restricting the inflow of fuel. Typically, these fill loss vapors are vented to atmosphere, either through a separate vent line, or out the end of the filler pipe, or both.
Proposed new regulations that would require the recovery of the fill loss vapors as well as the diurnal loss vapors have stimulated research and patent activity directed at new systems that would collect and store the fill loss vapors in the same canister. In the public debate over whether the responsibility for the recovery of fill loss vapors should rest with the automobile companies or with the oil companies, such systems have come to be referred to as "on board" systems, meaning that they are part of the vehicle, as opposed to being part of the filling station fuel pump apparatus. Several patents for various on board vapor recovery systems have issued already, and more certainly will in the future. Although they have not yet been generally adopted in production, the typical proposed on board system includes a seal in the filler pipe, and a valve, usually contained in a housing on the side of the filler neck, just below the open end thereof. A vent line runs from the valve housing to the canister. As the filler nozzle is inserted, it wipes through the seal, blocking the flow of vapors out of the end of the filler pipe, and also opens the valve by engaging some mechanism near the housing. Alternatively, removal of the cap alone opens the valve. After valve opening, fill loss vapors that would normally have exited the now blocked filler pipe are forced to vent through the valve housing and vent line to the canister. Other systems add a second line from the tank to the filler pipe that enters the filler pipe below the seal and valve housing, so that vapors displaced from the filling tank can flow out of the tank through the second line, into the filler pipe and then out, without having to pass the junction of the tank and filler pipe. A drawback of such systems is the number and expense of the various vent lines. Another potential drawback is in packaging. If the filler pipe is at the back of the tank, and the canister at the front, then the various lines running from the filler pipe to the canister will be long and heavy. Likewise, the valve housing may occupy a significant volume on the side of the filler pipe.